Display devices are being developed in which a light emitting element using an organic electroluminescence material is arranged in each pixel and an image is displayed by individually controlling the light which is emitted. A light emitting element includes a structure in which a layer having an organic electroluminescence material (also called an “organic layer”) is sandwiched between a pair of electrodes, one being distinguished as an anode electrode and the other as a cathode electrode. A display device formed with a pixel using such a light emitted element includes a pixel electrode in which one of the electrodes is arranged in each pixel, and a common electrode in which the other electrode is applied with a voltage common to a plurality of pixels. A display device displays an image by individually controlling the voltage of a pixel electrode in each pixel and setting the voltage of a common electrode as a fixed voltage.
A light emitting element arranged in each pixel is connected to a transistor. Since the transistor drives the light emitting element, it is called as a drive transistor. A drive transistor arranged in each pixel in a pixel region arranged with pixels is ideally preferred to have uniform characteristics. However, a plurality of drive transistors actually arranged in a pixel region has variation in electrical characteristics. At times, there is variation in a threshold voltage of a drive transistor and due to this, even when the same gate voltage is applied to a plurality of drive transistors, a problem arises whereby a drain current fluctuates. As a result, it is known that variation in the luminosity of a light emitting element occurs which degrades the display quality of an image. In order to solve such problems, a technology has been developed which compensates for a threshold voltage of a drive transistor arranged in each pixel (for example, see U.S. Patent Application No. 2006/0023551 specification, and U.S. Patent Application No. 2007/00262931 specification).